Microcellular synthetic bristles formed by the use of a blowing agent to form closed cells in the interior of the bristles and ruptured cells at the surface are known in the prior art, as exemplified by the teachings in U.S. Pat. No. 4,937,141 (Burns); U.S. Pat. No. 5,022,112 (Burns); U.S. Pat. No. 5,032,456 (O'Brien et al.) and U.S. Pat. No. 5,151,229 (Burns). The subject matter of all of the aforementioned patents is fully incorporated herein by reference.
Although the use of blowing agents in the manufacture of bristles has resulted in the formation of irregularities (i.e., provided by open cells) on the surface of the bristles, thereby simulating the desirable surface characteristics of natural bristles, the substantial randomness of such irregularities has made it difficult to provide a desired percentage of the bristles with a desired degree of such surface irregularity. Thus, a number of the formed bristles have surfaces that are so smooth, or have such extremely limited surface irregularities, that the desired benefits of a rough, irregular surface are not achieved.
Moreover, bristles formed only with blowing agents have not had the desired bend recovery characteristics for applications in which the bristles are subjected to high bending forces, and therefore crack, crimp, or often break, or often do not recover do to lack of stiffness.
In addition, although employing a blowing agent to form bristles does result in a lower bulk density than forming the bristles without such a blowing agent, a need still exists for bristles of even lower bulk density, but without detracting from other desirable characteristics of the bristles.
In addition, bristles formed solely with a blowing agent, or without a blowing agent, often are difficult to handle in brush forming equipment, due to the fact that the individual filaments do not easily flow relative to each other and therefore tend to move in clumps to the picker unit to interfere with the precise picking and placement of the bristles in the brushes.